1. Field of the Invention
This invention relates to a process for technically recovering terephthalic acid as the material of polyethylene terephthalate (PET), from spent PET, which is used as bottles containing beverages and the like and also to a system for use in such process.
2. Background of the Invention
Several methods have been proposed in each of which, PET is decomposed for recovering monomer components therefrom, and thus recovered monomer components can be used as the material of PET. Among these conventional methods, typical ones are listed below.
(1) Methanolysis method where depolymerization is carried out for PET with methanol in vapor phase or liquid phase for producing dimethyl terephthalate;
(2) Glycolysis method where depolymerization is carried out for PET with ethylene glycol for utilizing an intermediate product, i.e., bis hydroxyethyl terephthalate as the, material of polymer; or Ester interchange method where dimethyl terephthalate is obtained from the product of the Glycolysis method, i.e., bis hydroxyethyl terephthalate with methanol; and
(3) Hydrolysis method where hydrolysis is carried out for PET with alkali solution so that the resulting metallic salt terephthalate is neutralized with acid for the crystallization of terephthalic acid.
However, the above methods have drawbacks respectively.
In the method (1) (Methanolysis method), its reaction temperature is so low of about 177xc2x0 C., which requires a lot of reaction time.
The method (2) (Glycolysis method), like the method (1) (Methanolysis method), also requires a lot of reaction time. Additionally, it is difficult to depolymerize perfectly to recover the monomer components. Further, the intermediate product, i.e., bis hydroxyethyl terephthalate is partly dissolved into the ethylene glycol. Then, it is difficult to separate the dissolved bis hydroxyethyl terephthalate from the ethylene glycol, which results in poor yield.
In the method (2) (Ester interchange method), it is difficult to separate the ethylene glycol from the methanol. Further, since the dimethyl terephthalate is partly dissolved into the methanol, it is also difficult to separate the dissolved dimethyl terephthalate from the methanol.
In the method (3) (Hydrolysis method), it is more difficult to produce the terephthalic acid than the case of bis hydroxyethyl terephthalate and the case of dimethyl terephthalate. Further, the recovered terephthalic acid might be contaminated with various additives contained in the spent PET, which results in a problem.
To cope with the above problems, Japanese Published Unexamined Patent Application No. 286744/97 proposes a method, in which PET is decomposed with alkali in ethylene glycol.
However, the method disclosed in this patent publication has a problem because the terephthalic acid is not sufficiently recovered. Therefore, it is necessary to attain a method for recovering a polymer having high purity. Additionally, since these conventional methods are performed in pilot scales, there is an increased demand for establishment of practical and technical process of this kind.
It is therefore, the object of the present invention is to provide a technical and practical process of this kind with high recovery of terephthalic acid. The other objects of the present invention will be clear from the following explanation.
In accordance with the first aspect of the present invention, there is provided a process for recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, the process comprising the following steps;
(1) a decomposition reaction step where the pulverized products of spent polyethylene terephthalate arm continuously subjected to decomposition reaction in ethylene glycol in the presence of alkali, which is equimolar or excessmolar relative to the polyethylene terephthalate, so that salt of terephthalic acid and ethylene glycol can be obtained;
(2) a solid-liquid separation, dissolution, impurities-removing step where the ethylene glycol is separated from decomposition reaction slurry of the salt of terephthalic acid and ethylene glycol, and the solid salt of terephthalic acid is dissolved into water and insoluble impurities are removed;
(3) a neutralization, crystallization step, where the resulting solution of salt of terephthalic acid is neutralized with acid so that terephthalic acid can be crystallized;
(4) a solid-liquid separation, washing step where the resulting slurr of terephthalic acid crystals is subjected to solid-liquid separation so that the terephthalic acid crystals can be obtained and washed; and
(5) a drying, pulverization step where the washed terephthalic acid crystals are dried and pulverized.
According to the first aspect of the present invention, the terephthalic acid having high purity can be technically recovered with high efficiency from the pulverized products of spent polyethylene terephthalate.
The alkali comprising mainly sodium carbonate can be used as the above alkali and sulfuric acid can be used as the above acid. Due to the alkali comprising mainly the sodium carbonate, carbon dioxide is generated by the decomposition reaction, thus inlet gas is not required. Further when the sulfuric acid is used as the acid, the sodium sulfate (mirabilite) can be crystallized by the neutralization and recovered.
Water contained in liquid obtained by separation in the step (4) (the solid-liquid separation, washing step) is vaporized so that the sodium sulfate can be crystallized and separated. The resulting steam is condensed for re-using as the water for dissolving the salt of terephthalic acid, whereby the amount of required water can be decreased.
In the step (2) (solid-liquid separation, dissolution, impurities-removing step), by utilizing a crossfeed belt type vacuum filtering dissolver, the separation of the ethylene glycol, the dissolving of the solid salt of terephthalic acid, and the removing of the insoluble impurities can be continuously carried out.
The process in accordance with the first aspect of the present invention might further comprise the following steps:
(6) a return step where the ethylene glycol, which is obtained by the solid-liquid separation in the above step (2) (solid-liquid separation, dissolution, impurities-removing step) is returned to the step (1) (decomposition reaction step); and
(7) a vaporization, crystallization, separation, return step where water contained in separation liquid obtained from the step (4) (the solid-liquid separation, washing step) is vaporized so that the salt can be crystallized and separated, then the reminder, i.e., the ethylene glycol is returned to the step (1) (decomposition reaction step).
By returning the ethylene glycol, the amount of required ethylene glycol can be decreased.
The process in accordance with the first aspect of the present invention might further comprise, between the step (2) (solid-liquid separation, dissolution, impurities-removing step) and step (3) (neutralization, crystallization step), the following step;
(8) soluble impurities-removing step where soluble impurities, which are contained in solution in the above step (2), can be continuously removed with an adsorbent packed adsorber.
As water used for the dissolution in the above step (2), wash water of the adsorber in the step (8) and/or condensate obtained by cooling steam in the step (7) can be re-used for saving water.
Between the above step (1) (decomposition reaction step) and step (2) (solid-liquid separation, dissolution, impurities-removing step), the decomposition reaction slurry can be passed through orifice means. Due to this orifice means, the ethylene glycol can be partly prevented from being passed to the step (2). This means that the slurry having small amount of ethylene glycol is passed to the step (2). Accordingly, load applied to the step (2) can be lowered.
The above orifice means might comprise: a tapered cylinder and a screw conveyor placed in the cylinder. The orifice means of this kind can be easily formed by providing tapered cylinder and the screw conveyor placed in an outlet portion of a screw press.
In the above step (1) (decomposition reaction step), a horizontal type decomposition reactor can be used. This reactor comprises: a cylinder, which has a body portion and a tapered portion connected to the fore end portion of the body portion; and a screw conveyor, which is placed in the cylinder and which has a tapered portion corresponding to the above tapered portion of the cylinder. Then, the pulverized products of spent polyethylene terephthalate, ethylene glycol, and alkali are charged into the above horizontal type decomposition reactor. In this reactor, the ingredients are subjected to the decomposition reaction, mainly in the body portion, so that the salt of terephthalic acid and the ethylene glycol can be obtained. Finally, the resulting decomposition reaction slurry can be extruded from the tapered portion of the cylinder.
For the neutralization of the step (3) (neutralization, crystallization step), the above solution of salt of terephthalic acid can be fed into a neutralization chamber, where the solution is subjected to acid jetting. Then, acid jetting point is moved peripherally with the passage of time. By such movement of jetting point, the neutralization can be promoted in the neutralization chamber, which enables complete neutralization while any unreacted acid is not remained.
For the neutralization of the step (3) (neutralization, crystallization step), ultrasonic vibration can be applied in the neutralization chamber. Also due to the ultrasonic vibration, the neutralization can be promoted, which enables complete neutralization while any unreacted acid is not remained.
For the drying of the step (5) (drying, pulverization step), indirect heating is performed by heating medium passing through a jacket and/or an agitating blade means under reduced pressure of inside of a container for the drying. By drying the terephthalic acid crystals in this way, the degeneration of terephthalic add can be prevented, and the terephthalic acid crystals can be dried in short time.
For the washing of the step (4) (solid-liquid separation, washing step), the slurry of terephthalic acid crystals is supplied to a running filter cloth. Then, this filter cloth is divided into a plurality of filtering and washing zones along the running direction of the filter cloth. In one filtering and washing zone, wash water is sprayed and the slurry is filtered by vacuum suction so that filtrate is obtained under this filtering and washing zone. This filtrate is used as a wash water in another filtering and washing zone located at the upstream side of the above filtering and washing zone, that is to say, counter-flow type washing is carried out. In this embodiment, the filtering and washing can be performed efficiently.
In accordance with the second aspect of the present invention, there is provided a process for recovering terephthalic acid from pulverized products of spent polyethylene terephthalate, the process comprising the following steps;
(1) a decomposition reaction step where the pulverized products of spent polyethylene terephthalate are subjected to decomposition reaction in ethylene glycol in the presence of alkali, which is equimolar or excessmolar relative to the polyethylene terephthalate, so that salt of terephthalic acid and ethylene glycol can be obtained;
(2) a solid-liquid separation step where the ethylene glycol is separated from decomposition reaction slurry of the salt of terephthalic acid and the ethylene glycol;
(3) a dissolution step where the salt of terephthalic acid, which is obtained by separating ethylene glycol from the slurry, is dissolved into water;
(4) a neutralization, crystallization step where the resulting solution of salt of terephthalic acid is neutralized with acid so that terephthalic acid can be crystallized;
(5) a solid-liquid separation, washing step where the slurry of terephthalic acid crystals is subjected to solid-liquid separation so that the terephthalic acid crystals can be obtained and washed; and
(6) a drying, pulverization step where the washed terephthalic acid crystals are dried and pulverized.
As shown in the second aspect of the present invention, even if the step (2) (solid-liquid separation step) and the step (3) (dissolution step) are performed separately, the directing terephthalic acid can be obtained smoothly.
In the above step (1) (decomposition reaction step), a vertical type stirring vessel can be used for the decomposition reaction. When operation is passed from the step (1) to the step (2), the decomposition reaction slurry can be passed through a vertical type orifice means. This orifice means comprises a tapered cylinder and a screw conveyor placed in this cylinder. Due to this orifice means, the ethylene glycol can be partly prevented from being passed to the step (2). On the other hand, the ethylene glycol can be returned to the stirring vessel from the larger diameter side of the above cylinder. Thus, the slurry containing small amount of ethylene glycol can be passed to the step (2). Accordingly, load applied to the step (2) can be lowered.
In accordance with the third aspect of the present invention, there is provided a process comprising a decomposition reaction step where pulverized products of spent polyethylene terephthalate are subjected to decomposition reaction in solvent in the presence of alkali, which is equimolar or excessmolar relative to the polyethylene terephthalate, so that salt of terephthalic acid and ethylene glycol can be obtained, for recovering terephthalic acid from the resulting salt of terephthalic acid, and the above decomposition reaction step being constructed from equal to or more than two stages, i.e., multiple stages.
The pulverized products of spent PET are formed by crushing PET bottles or the like. Then, each PET article of this kind has many parts made of different materials, thus time required for decomposition reaction is different for each part. Accordingly, in order to decompose completely the pulverized products of spent PET, it takes time long enough to decompose a part where the decomposition reaction is slowest among all the parts. Precisely, even after the decomposition reaction is completed at a certain part, this process can not move to next step as long as decomposition reaction is occurred at any part. This causes low efficiency.
In this connection, in the third aspect of the present invention, since the decomposition reaction step is constructed from equal to or more than two stages, i.e., multiple stages, the decomposition reactions in the parts of the pulverized products are performed in corresponding stages, respectively, so that the reactions in these all parts ran be finished simultaneously, resulting in improved efficiency of the process.
As one embodiment in accordance with this aspect of the present invention, the above decomposition reaction step might comprise two stages; a previous heating stage and a decomposition reaction heating stage. In the previous heating stage, the above pulverized products are previously heated for 5 minutes or more at the temperature range from 100xc2x0 C. to the temperature at which the decomposition reaction of the above pulverized products is not substantially started. On the other hand, in the decomposition reaction heating stage, the above pulverized products are subjected to the decomposition reaction at the temperature range from the temperature at which the decomposition reaction of the above pulverized products is substantially started and the boiling point of the solvent.
As the solvent, ethylene glycol can be used and the above pious heating can be performed at the temperature of 100 to 140xc2x0 C., while the above decomposition reaction can be performed at the temperature of 130 to 180xc2x0 C.
As stated above, as for the previous beating stage, where the above pulverized products are previously heated for 5 minutes or more at the temperature range from 100xc2x0 C. to the temperature at which the decomposition reaction of the above pulverized products is not substantially started, and as for the decomposition reaction heating stage where the above pulverized products are subjected to the decomposition reaction at the temperature range from the temperature at which the decomposition reaction of the above pulverized products is substantially started to the boiling point of the solvent, particularly, if the ethylene glycol is used as the solvent, the above previous heating temperature is 100 to 140xc2x0 C., while the above decomposition reaction temperature is 130 to 180xc2x0 C. By performing the previous heating before the decomposition reaction, the amorphous pars of the pulverized products of spent PET are crystallized and subjected easily to the decomposition reaction. As a result, the decomposition reactions in these all parts can be finished simultaneously, resulting in improved efficiency of the process in the decomposition reaction stage.
Further, before the previous heating stage, impurities, each of which has smaller specific gravity than that of the solvent, are preferably removed.
Generally, tee pulverized products of spent PET contain impurities such as polypropylene (PP) and polyethylene (PE). When the impurities are heated, they will surely melt. Hence, it is difficult, after heating, to remove such impurities. Then, if the impurities are not removed but remained, they will solidify when they are cooled. This might cause trouble in the other steps continuing after the decomposition reaction step. In order to avoid such trouble, it is preferable that the impurities such as PP and PE are removed before heating. Actually, the impurities float on the solvent such as ethylene glycol and water, because of smaller specific gravity than that of the solvent. Therefore, it is surely possible that the floating impurities are removed from the solvent before heating. Particularly, when the ethylene glycol is used as the solvent, such impurities can be removed easily, due to the large difference of specific gravity between the impurities and the ethylene glycol.
As a decomposition reactor used for a final stage of the above decomposition reaction step, a horizontal type decomposition reactor can be used. This reactor comprises: a cylinder, which has a body portion and a tapered portion connected to the fore end portion of the body portion; and a screw conveyor, which is placed in the cylinder and which has a tapered portion corresponding to the above tapered portion of the cylinder. Then, the pulverized products of spent polyethylene terephthalate, solvent, and alkali are charged into this horizontal type decomposition reactor. Continuously, the above pulverized products are subjected, mainly in the above body portion, to the decomposition reaction so that the salt of terephthalic acid and the ethylene glycol can be obtained. Finally, the resulting decomposition reaction slurry can be extruded from the tapered portion of the cylinder.
When the decomposition reaction slurry goes to the next step, since the slurry is extruded from the tapered portion as stated above, large amount of solvent is not required to be included in the slurry. Accordingly, the load applied to the next step can be lowered. Additionally, such extrusion is carried out continuously after the decomposition reaction in the horizontal type decomposition reactor. Therefore, any additional equipment for the extrusion is not required.
The sodium carbonate is preferably used as the alkali. Additionally, it is preferable that the mass ratio of the pulverized products of spent polyethylene terephthalate and the ethylene glycol is determined to be 1:0.8 to 1.2 in the previous heating and to be 1:2.0 to 2.5 in the decomposition heating.
When the sodium carbonate is used as the alkali, the pulverized products of spent PET are subjected to the decomposition reaction so that the sodium terephthalate and the ethylene glycol can be obtained. In this situation, the sodium terephthalate absorbs the ethylene glycol. Then, as stated above, in a certain part where the decomposition reaction is fast, even after the decomposition reaction of this part is completed, the reaction is continued due to the part where the decomposition reaction is slowest That is to say, after the decomposition reaction of this part is completed, sodium terephthalate continues to absorb the ethylene glycol as long as the decomposition reaction is occurred in other parts. This requires large amount of ethylene glycol.
In this connection, in the fourth aspect of the present invention, by provision of the previous heating, the decomposition reactions of the all parts of the pulverized products of spent PET can be finished simultaneously. Accordingly, the amount of ethylene glycol absorbed into the sodium terephthalate can be deceased. Then, if the mass ratio of the pulverized products of spent PET and the ethylene glycol is determined to be 1:0.8 to 1.2 in the previous heating, and it is determined to be 1:2.0 to 2.5 in the decomposition heating, the reactions can be carried out efficiently in both cases. As a result, the consumption of the ethylene glycol can be decreased. Here, the amount of ethylene glycol in the decomposition reaction heating refers to not the amount of ethylene glycol added in the decomposition reaction stage, but the total of the amount of ethylene glycol added in the previous heating stage and the amount of ethylene glycol added in the decomposition reaction stage.
In accordance with the fourth aspect of the present invention, there is provided a process comprising decomposition reaction heating carried out for the pulverized products of spent polyethylene terephthalate in solvent in is the presence of alkali, which is equimolar or excessmolar relative to the polyethylene terephthalate, so that salt of terephthalic acid and the solvent can be obtained, for recovering terephthalic acid from the resultant salt of terephthalic acid, and before the decomposition reaction, the above pulverized products are subjected to thermal degradation.
As stated above, each PET article such as PET bottle for beverage has many parts made of different materials, b, time required for decomposition reaction is different for each part. Accordingly, even after decomposition reaction is completed at a certain part, operation can not be passed to the next step as long as decomposition reaction is occurred at any part. In this connection, in accordance with the fourth aspect of the present invention, before the above decomposition reaction, the pulverized products of spent PET are subjected to thermal degradation so that the decomposition reactions in the all parts are finished simultaneously. Particularly, if this thermal degradation is performed at the temperature of 290 to 330xc2x0 C. for more than 5 minutes, the amorphous parts of the pulverized products of spent PET are crystallized and subjected easily to the decomposition reaction As a result, the decomposition reaction can be finished quickly, resulting in high efficiency.
The thermal degradation can be carried out continuously and easily by utilizing a screw extruder.
Further, heat generated by the pulverized products subjected to the thermal degradation can be recycled for the decomposition reaction heating. This means that heat can be used efficiently, thereby efficient process can be attained. After the thermal degradation of the pulverized products, they are fed to the decomposition reaction step, where they are heated. Then, the amount of the fed pulverized products can be adjusted. By doing so, temperature control in this step is enabled.
In accordance with the fifth aspect of the present invention, there is provided a process for technically recovering terephthalic acid from pulverized products of spent polyethylene terephthalate. This process comprises: a thermal cracking course where the pulverized products of spent polyethylene terephthalate are subjected to thermal cracking in the presence of solvent and alkali so that salt of terephthalic acid and ethylene glycol can be obtained; and a removing course where the solvent is removed from thermal cracking slurry, by vaporizing the solvent under the atmospheric pressure or reduced pressure from the thermal cracking slurry, for recovering terephthalic acid from the resulting salt of terephthalic acid.
In order to remove the solvent from the thermal cracking slurry, i.e., to separate the thermal cracking slurry into the salt of terephthalic acid and the solvent, it is common that a filter or centrifugal separator is used. However, if such equipment is used, since the particles of the salt of terephthalic acid are very fine, the amount of the removed solvent can not be stable. Additionally, the removing operation takes long time. On the other hand, in this aspect of the present invention, the solvent is vaporized under the atmospheric pressure or reduced pressure. Accordingly, the amount of the removed solvent can be controlled freely and the removing operation can be carried out quickly.
Further, in this aspect of the present invention, there is provided a process for technically recovering terephthalic acid from pulverized products of spent polyethylene terephthalate. This process comprises: a thermal cracking course where the pulverized products of spent polyethylene terephthalate arc subjected to thermal cracking in the presence of solvent and alkali so that the salt of terephthalic acid and ethylene glycol can be obtained; and a removing course where the solvent is removed from the thermal cracking slurry, by vaporizing the solvent under the atmospheric pressure or reduced pressure until the amount of the solvent contained in the solid salt of terephthalic acid is decreased to 20 to 30% by mass thereof, for recovering the terephthalic acid by dissolving the solid salt of terephthalic acid into water.
In this embodiment, the solvent and the ethylene glycol are removed from the solid salt of terephthalic add until the amount of the solvent contained in the solid salt of terephthalic acid is decreased to 20 to 30% by mass thereof. Therefore, the amount of water required to dissolve the solid salt of terephthalic can be decreased due to the certain amount of solvent contained in the solid salt of terephthalic acid.
To carry out the process according to this embodiment, there is provided a system recovering terephthalic acid from pulverized products of spent polyethylene terephthalate. This system comprises: a horizontal type decomposition reaction chamber; stirring means for stirring solvent, alkali, and the pulverized products of spent polyethylene terephthalate, which are charged into the decomposition reaction chamber; heating means for heating the same; a vacuum chamber, which is communicated with the upper portion of the above decomposition reaction chamber; and evacuating means, which is connected to the vacuum chamber. Then, in this system, the solvent contained in the above decomposition reaction chamber is removed through the above vacuum chamber, and the resulting solid salt of terephthalic acid will be used for recovering the terephthalic acid.
This system has the vacuum chamber, which is communicated with the upper portion of the decomposition reaction chamber, and evacuating means, which is connected to this vacuum chamber. Then, by forming a vacuum in the decomposition reaction chamber, the solvent can be vaporized. That is to say, the decomposition reaction chamber can be used not only for the decomposition reaction but also for the removing of solvent. Therefore, any additional equipment is not required for removing the solvent. Additionally, in this system, beat can be used efficiently.
In this aspect of the present invention, carbonate can be used as the alkali. When the carbonate is used as the alkali, carbon dioxide is generated the moment the decomposition reaction is started. Accordingly, the decomposition reaction chamber can be configured so as to be sealed internally by thus generated carbon dioxide. In this case, inert gas (e.g, nitrogen) is not required for this sealing.
Further, according to this embodiment, there is provided a system recovering terephthalic acid from pulverized products of spent polyethylene terephthalate. This system comprises: a decomposition reactor where the pulverized products of spent polyethylene terephthalate are subjected to thermal cracking in the presence of solvent and alkali so that salt of terephthalic acid and ethylene glycol can be obtained; an evaporator having a heating surface kept at the temperature equal to or higher than the boiling point of the above solvent; and falling means, by which the above thermal cracking slurry is fallen downward to the heating surface. Then, in this system, the solvent vaporized in the above evaporator is recovered axed returned to the above decomposition reactor.
In this system, by the falling means, the thermal cracking slurry of the salt of terephthalic acid and ethylene glycol can be brought into contact with the heating surface kept at the temperature equal to or higher than the boiling point of the above solvent, the solvent can be vaporized quickly. Further, the temperature of the heating surface can be adjusted so that the amount of removed solvent can be controlled freely.
In accordance with the sixth aspect of the present invention, there is provided a process for technically recovering terephthalic acid from pulverized products of spent polyethylene terephthalate. This process comprises: subjecting the pulverized products of spent polyethylene terephthalate to thermal cracking in the presence of alkali so that salt of terephthalic acid and ethylene glycol can be obtained; removing the ethylene glycol from thermal cracking slurry so that solid salt of terephthalic acid can be obtained; dissolving the solid salt of terephthalic acid into water so that solution of salt of terephthalic acid can be obtained; neutralizing this solution of salt of terephthalic acid by adding acid so that terephthalic acid can be crystallized and recovering the terephthalic acid. Then, in this process, the acid is added in multiple stages and the amount of added acid is determined so that the pH value of the solution in the final stage is adjusted to 2 to 4. Further, in this process, the terephthalic acid crystals obtained from the fin stage are returned to a previous stage and dissolved.
For crystallizing the terephthalic acid, it is usually considered that solution of salt of terephthalic acid is neutralized by adding acid until the pH value of the solution is adjusted to about 2. However, in this case, the particle size of each resulting terephthalic acid crystals is too small. To cope with this problem, there is provided a method where the acid is added to the solution of salt of terephthalic acid under high pressure at the temperature of 100 to 200xc2x0 C. so that the terephthalic acid crystals each having large particle size can be obtained. However, this method makes the facility to be complicated, resulting in high cost. Further, since thus obtained terephthalic acid might form capillary crystals, the crystals tend to break. As a result, the terephthalic acid crystals each having the large particle size can not be obtained by the conventional methods.
On the other hand, according to the process of this aspect, the terephthalic acid crystals each having large particle size can be obtained at low cost.
It is preferable that the amount of added acid is determined so as not to crystallize the terephthalic acid in the proceeding stage.
It is also preferable that the acid is added in multiple stages while the amount of added acid is determined so that the pH value of the solution obtained from the final stage is adjusted to 2 to 4 and this solution is fed into a classifier so that classified terephthalic acid fine crystals are returned to a previous stage together with mother liquor. This embodiment ensures the terephthalic acid laving the large particle size.
It is possible that the solution containing the terephthalic acid crystals is subjected to solid-liquid separation so that the resulting terephthalic acid crystals are returned to the previous stage for dissolution again.